The present invention relates to a copier, facsimile apparatus, printer or similar image forming equipment capable of producing a multicolor image and, more particularly, to color image forming equipment of the type forming a multicolor image on an image carrier and then transferring it to a recording medium.
Image forming equipment of the type repeating charging, exposure and development with an image carrier to form a multicolor image on the image carrier and then transferring it to a recording medium by a single transfer has been proposed in various forms in the past.
For example, Japanese Patent Laid-Open Publication No. 60471/1988 discloses image forming equipment which has a plurality of developing means arranged around an image carrier to produce a multicolor image. Developing means located at the downstream side uses a toner opposite in polarity to the toner of the preceding or upstream developing means. A toner layer for development in the succeeding or downstream developing means does not contact the image carrier. While an AC electric field for depositing the toner is applied when the succeeding developing means is in operation, an electric field for depositing the toner of the preceding developing means is applied when the succeeding development means is not operated. The toner layer in the succeeding developing means is selected to be 30 .mu.m to 500 .mu.m thick. The preceding developing means is loaded with a two-component developer containing a toner of chromatic color, while the succeeding developing means is loaded with a one-component developer implemented as a black toner.
Japanese Patent Laid-Open Publication No. 63061/1988 teaches an image recording method using a photoconductive element having a 35 .mu.m to 90 .mu.m thick photoconductive layer made of selenium or arsenic selenade and having a capacitance of 20 pF to 170 pF. A plurality of developing means are arranged around the photoconductive element. The equipment performs charging, exposure and development a plurality of times to form color images on the same photoconductive element. Each developing means is spaced apart by less than 250 .mu.m from the photoconductive element. During development, a thin toner layer does not contact the photoconductive element, and a DC bias for development is applied. The photoconductive element has a 15 .mu.m to 50 .mu.m thick organic photocondutive layer. The equipment effects reversal development. The image portion and a non-image portion have a potential contrast higher than 400 V. The thickness of a toner layer formed on the photoconductive element by development in one color ranges from 5 .mu.m to 30 .mu.m. The surface of the photoconductive element is charged by a scorotron charger.
Further, Japanese Patent Laid-Open Publication 85578/1988 proposes image forming equipment in which a plurality of developing means are located to face a recording medium without contacting the later so as to produce a multicolor image. The first developing means located at the upstream side is rotated in the same direction as and at a higher speed than the recording medium, and it is assigned to development in black. A DC-biased AC bias is applied to the first developing means. The second and successive developing means downstream of the first developing means each effects equispeed development in color and is applied with a DC bias only.
When a multicolor image is to be formed on an image carrier, the developments in the second and successive colors are implemented as non-contact developments. Preferably, use should be made of an image forming system using a non-magnetic toner to facilitate color image formation, especially a single-component system which promotes a miniature and inexpensive configuration. The one-component system effects development by causing a toner layer to face an image carrier without contacting the latter. In the event of development in second color, a toner image has already been formed on the image carrier. Hence, should an AC bias be applied as a bias for development a in Japanese Patent Laid-Open Publication No. 60471/1988, the toner would move back and forth between the latent image surface and the surface of the toner carrier. This not only disturbs the toner image of first color but also causes the toner of first color to enter the developing means loaded with a toner of second color to thereby render the latter impure.
To eliminate the above problem, the developments in the second and successive colors may cause a non-magnetic toner to fly by a DC electric field, as taught by the Japanese Patent Laid-Open Publication Nos. 63061/1988 and 85578/1988. However, a toner layer cannot be transferred by a DC electric field unless the charge of the toner is controlled to be small enough to orient the electric field acting on the toner layer such that it causes the toner to fly. From the efficient flight standpoint, a toner layer whose amount of charge is less than 5 .mu.C/g as measured by a suction method. However, a toner with such a small amount of charge does not fly faithfully along the electric field, reducing the clear-cutness of the resulting image.
Another problem with the fight flight relying on an DC electric field is that the toner does not fly until an electric field causing the toner to begin to fly, i.e., a threshold electric field, has been developed in a developing region. Since the toner layer suddenly begins to fly at the threshold electric field, the resulting image suffers from a poor gamma characteristic, i.e., it loses portions of low contrasts.
Further, the electrostatic restraint acting on the toner with a small amount of charge is weak and, therefore, causes it to be scattered around to smear the interior of the equipment.
The adequate range of Q/M of a toner for development is 5 .mu.C/g to 30 .mu.C/g. To cause a toner with such a strong mirror image force to fly, it is necessary that the van der waals' forces acting on the toner carrier surface together with the mirror image force be reduced. Then, the degree of cohesion of the toner has to be confined in the range of 5% to 30%. This, however, allows the toner to move easily on the toner carrier and makes it difficult to form two or more toner layers. When development is effected with a single toner layer being moved at substantially the same speed as a latent image, the resulting image density if low. Should the speed ratio be doubled or tripled, the density would be higher at the trailing edge of the resulting image than at the leading edge. Such an irregular density distribution is espetially critical from the tonal aspect when it comes to a color image.